This proposed project will involve efforts to determine the control elements for regulation of aminoacyl-tRNA synthetase formation in both Escherichia coli and Salmonella typhimurium, and the elucidation of the exact role(s) of these enzymes in repression control of the corresponding biosynthetic operons. The principal methods of study will involve the use of genetically based and/or physiologically imposed altered repression control responses in cells cultivated under specific growth conditions, along with purification of the cellular components presumed to be critical for control and the performance of appropriate in vitro tests of the capacities of such elements to generate the repression control signals for the aminoacyl-tRNA synthetases and the cognate biosynthetic enzymes.